The overall objective of this research is to determine whether there is a time-dose relationship in the response of biologic systems to irradiation by clinically important radionuclides decaying at different rates. Large amounts of short half-lived radionuclides are now given to patients, yielding better diagnostic tests without increasing the radiation dose. This research is intended to determine whether there is a time-dose relationship such that the short-life (with its high dose-rate) may actually produce more damage than the same dose of radiation delivered at a slower rate by a longer-lived radionuclide. The regenerating rat liver was chosen as a model system in which to study the effects of X-ray administered essentially instantaneously and by radioactive colloids of differing half-lives. Our specific aims for the total project period are: to compare the biological effects of the same radiation dose delivered at different rates; to determine if differing dose-rates will produce differences in later effects and persistence of chromosomal damage; and determine if damage produced by radiations delivered at increased dose-rates will result in increased incidence or shortened latent period of hepatic tumors.